The present invention relates in general to the field of high-speed telecommunications, and in particular to integrated transceivers that include a voltage-controlled oscillator (VCO) in the transmitter and another VCO in the receiver portion of the transceiver.
A single integrated circuit acting as a transceiver combines both a transmitter and a receiver on a single die. Such an integrated circuit has the advantages of reducing the communication system cost and providing easier diagnostic loop-back functions. A phase-locked loop (PLL) is an extremely useful system that is commonly used in a number of different applications and especially in communication systems. The heart of a PLL is a voltage-controlled oscillator (VCO) whose ideal output is a periodic signal the frequency of which is controlled by an input voltage.
In the context of a transceiver, analog PLL technology has been used for both transmitter and receiver circuits due to its superior performance in terms of jitter and overall accuracy. Typically, a transmitter transmits data using a clock signal that is locked to an output frequency of a local reference clock generated by, e.g., a local crystal. Thus, to recover the data, a receiver must lock to the frequency of the remote transmitter from which it receives its data. In an integrated transceiver that includes both a transmitter and a receiver on the same substrate, the VCO of the receiver locks to the frequency of a remote oscillator, while the VCO of the transmitter locks to the frequency of a local oscillator. Often the local and remote oscillators run at slightly different frequencies. Crosstalk between the two PLLs that run at different frequencies causes intermodulation which will degrade the transceiver's performance. Intermodulation refers to the condition in which one VCO tracks not only its own reference frequency but the frequency of the other VCO. There are a number different sources for crosstalk and other types of noise in an integrated transceiver that give rise to intermodulation. These include crosstalk through the common substrate, coupling through the power supply, or coupling caused by radiation. For example, both the jitter characteristic of the receiver and the jitter characteristic of the transmitter are affected by the undesirable crosstalk between the transmitter and receiver that occurs when the transmitter and receiver are placed on the same integrated circuit. For very high-speed applications, the degradation of the jitter performance of the transceiver caused by intermodulation can cause either the transmitter, receiver, or both to be unable to meet the specification.
As is apparent from the above discussion, there is a need for a reducing the undesirable effects of intermodulation that occurs when operating a single-die transceiver.